The present invention relates to a high-speed and highly accurate automatic focusing apparatus.
An automatic focusing method is roughly divided into two methods one is a passive method and the other is an active one. Among these methods, an infrared active method has been becoming popular rapidly for a camera of a lens shutter type. The reason behind this is that an automatic focusing function works effectively even in a dark place and it is not affected by the contrast and pattern of a subject. In addition to that, the infrared active method is relatively simple in terms of operation and is more advantageous in terms of cost than other methods.
FIG. 17 shows a principle of an infrared active phase difference method. In this method, a projector lens emitted from infrared LED 1 is projected through projector lens 2 on subject 3. Reflected light from subject 3 is received by photoreceptor lens 4 and then is led thereby to sensor 5 composed of quartered SPD. In this case, the position of sensor 5 for receiving light changes depending upon the distance between a camera and a subject. Therefore, the difference in position of the sensor for receiving light tells a camera-to-subject distance.
However, this infrared active phase difference method has its disadvantage that a parallax is caused because a range-finding portion (including infrared LED 1, projector lens 2, photoreceptor lens 4 and sensor 5) is seperated from a camera lens.
On the other hand, FIG. 18 represents a diagram showing a principle of an infrared active method of a TTL type developed for improving aforesaid disadvantage of the infrared active phase difference method. In this method, infrared light from infrared LED 1 is reflected on beam splitter 6 and is transmitted by lens system 7 to arrive at subject 3. Reflected light from subject 3 is condensed by photoreceptor lens 8 to be led to photoreceptor sensor 9 which is illustrated in detail in FIG. 19. In this case, a movement of photoreceptor sensor 9 causes a quantity of accepted light at image-forming spot 9c to be the maximum. The position of photoreceptor sensor 9 in this case tells a camera-to-subject distance. In this method, no parallax is caused because infrared light for range-finding is projected from a camera lens, which is an advantage.
However, a photoreceptor portion for range-finding (including a photoreceptor lens and a photoreceptor sensor) is required, which prevents a camera from being compact size and low cost. In addition to that, a scope of range-finding is very narrow because a projecting point of infrared light is constantly fixed, therefore, the range-finding for the outside of an irradiation angle of infrared light is impossible, which is a disadvantage.